How to filter X -> 0 or X -> 1 from signals for events

Hi all,

          I am having problem in filtering the changes from x->1 and x->0 on a singal which i use to generate events.

       For example,

            I have a simple port "a" of type in, which is connected to a DUT signal. DUT drives x on the signal "a" for some time initially

 (10ns)   and then drives the value 1. I declared an event as shown below. clk_r is a clock rising event. The first clk_r event is available

  well after signal "a" settles with the value 1.

      event a_e is rise(a$)@clk_r; 

   Because of the change from x->1 on the signal "a", i receive an event on a_e on the first clk_r event.

   Can anyone suggest me how i can filter this unwanted event.